util: remove some no-op thread functions

Neither virThreadInitialize or virThreadOnExit do anything since we
dropped the Win32 threads impl, in favour of win-pthreads with:

  commit 0240d94c36
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Wed Jan 22 16:17:10 2014 +0000

      Remove windows thread implementation in favour of pthreads

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemumigparamstest.c

@@ -209,8 +209,7 @@ mymain(void)
     return EXIT_AM_SKIP;
 #endif
 
-    if (virThreadInitialize() < 0 ||
-        qemuTestDriverInit(&driver) < 0)
+    if (qemuTestDriverInit(&driver) < 0)
         return EXIT_FAILURE;
 
     virEventRegisterDefaultImpl();